1. Field of the Invention
The present invention relates to a liquid crystal display and a driving method thereof. More particularly, the present invention relates to a field sequential driving type of liquid crystal display (LCD) and a driving method thereof.
2. Description of the Related Art
As personal computers and televisions, etc., have become more lightweight and thin, the demand for lightweight and thin display devices has increased. According to such requirements, flat panel displays such as LCDs have recently been developed for use instead of cathode ray tubes (CRT).
An LCD is a display device used to display images corresponding to a desired video signal by applying electric fields to liquid crystal materials having an anisotropic dielectric constant and injected between two substrates, and controlling the strength of electric fields so as to control an amount of light from an external light source (i.e., backlight) transmitted through the substrates.
The LCD is representative of portable flat panel displays, and TFT-LCDs using a thin film transistor (TFT) as a switch are mainly used.
Each pixel in the TFT-LCD can be modeled with a capacitor having liquid crystal as a dielectric substance, such as a liquid crystal capacitor. An equivalent circuit of each pixel in such an LCD is shown in FIG. 1.
As shown in FIG. 1, each pixel of an LCD includes a TFT 10, of which a source electrode and a gate electrode are respectively connected to a data line Dm and a scan line Sn, a liquid crystal capacitor Cl connected between a drain electrode of the TFT 10 and common voltage Vcom, and a storage capacitor Cst connected to the drain electrode of the TFT 10.
In FIG. 1, when a scan signal is applied to the scan line Sn and the TFT 10 is turned on, data voltages Vd supplied to the data line Dm are applied to a pixel electrode (not shown) though the TFT 10. Then, an electric field corresponding to a difference between pixel voltages Vp applied to pixel electrodes and the common voltage Vcom is applied to liquid crystal (which is equivalently shown as the liquid crystal capacitor Cl in FIG. 1). Light transmits with a transmittivity corresponding to the strength of the electric field. In this instance, a pixel voltage Vp is maintained during one frame or one field, so that the storage capacitor Cst in FIG. 1 is used to maintain the pixel voltage Vp applied to the pixel electrode.
Generally, methods for driving an LCD can be classified into two methods, which are a color filter method and a field sequential driving method, based on methods of displaying color images.
An LCD using a color filter method has color filter layers composed of the three primary colors of red R, green G, and blue B in one of two substrates, and displays a desired color by controlling amount of lights transmitted through the color filter layers. An LCD using a color filter method controls an amount of light transmitted through the R, G, and B color filter layers when light from a single light source is transmitted through the R, G, and B color filter layers, and uses the R, G, and B color lights to display a desired color.
An LCD device for displaying color using a single light source and thee color filter layers uses unit pixels that respectively correspond to R, G, and B subpixels, thus at least three times the number of pixels are needed compared to displaying black and white. Therefore, fine manufacturing techniques are required to produce video images having high definition.
Further, there are problems in that separate color filter layers must be formed on a substrate for an LCD during manufacturing, and the light transmission rate of the color filters must be improved.
On the other hand, a field sequential driving type LCD sequentially and periodically turns on independent light sources of R, G, and B colors, and adds synchronized color signals corresponding to each pixel in accordance with the periodic turning on of lights to obtain full colors. That is, according to a field sequential driving type of LCD, one pixel is not divided into R, G, and B sub pixels, and lights of three primary colors outputted from R, G, and B backlights are sequentially displayed in a time-divisional manner so that the color images are displayed using an after image effect of the eyes.
The field sequential driving method can be classified as an analog driving method or a digital driving method.
The analog driving method establishes a plurality of gray scale voltages, selects one gray scale voltage corresponding to gray scale data from among the gray scale voltages, and drives a liquid crystal panel with the selected gray scale voltage to perform gray scale display with an amount of transmission corresponding to the gray scale voltage applied.
FIG. 2 shows a driving voltage and amount of light transmission of a conventional LCD using the analog driving method.
Referring to FIG. 2, a driving voltage having a V11 level is applied to the liquid crystal, and light corresponding to the driving voltage having the V11 level is transmitted through the liquid crystal in the R field period Tr for displaying an R color. A driving voltage having a V12 level is applied to the liquid crystal, and light corresponding to the driving voltage having the V12 level is transmitted through the liquid crystal in the G field period Tg for displaying a G color. Further, a V13 level driving voltage is applied to the liquid crystal, and an amount of light transmission corresponding to the V13 level is obtained. As such, a desired color image is displayed by a combination of R, G, and B lights transmitted respectively during the Tr, Tg, and Tb field periods.
With reference to FIG. 2, a period for displaying R color is the period Tr in the range of the time t1 to t2 in which R backlight emits the light; a period for displaying G color is the period Tg in the range of the time t3 to t4 in which G backlight emits the light; and a period for displaying B color is the period Tb in the range of the time t5 to t6 in which B backlight emits the light.
On the other hand, a digital driving method applies a constant driving voltage to the liquid crystal, and controls the voltage applying time to perform a gray scale display. The digital driving method maintains a constant driving voltage, and controls timing of a voltage applying state and a voltage non-applying state, so as to control a total amount of light transmitted through the liquid crystal.
FIG. 3 shows a waveform which illustrates a driving method of an LCD of a conventional digital driving method, and shows a waveform of a driving voltage and optical transmittivity of liquid crystal based on driving data having a predetermined number of bits.
Referring to FIG. 3, gray scale waveform data corresponding to each gray scale is provided with a digital signal having a predetermined number of bits, for example, a 7 bit digital signal, and a gray scale waveform according to 7 bit data is applied to the liquid crystal. Optical transmittivity of the liquid crystal is determined based on the gray scale waveform applied to perform gray scale display.
Meanwhile, researches have been undertaken to realize mild images (i.e., images having more smooth transitions of gray scale levels or pixel intensities) by displaying various gray scales during a limited time.